1. Field of the Invention
The invention relates to a measuring arrangement for determining at least one measured quantity with at least one sensor device and at least one transmitter device. Here, the sensor device generates at least one measurement signal and the transmitter device generates at least one output signal at least from the at least one measurement signal. The measured quantity is, for example, a flow rate, a liquid level, a pH value or the temperature of a medium.
2. Description of Related Art
In modern process automation, a plurality of measuring devices is used for determining or monitoring measured quantities in order to monitor or control processes. If the applications are especially risky or critical to safety, the measuring devices, or in general measuring arrangements, must satisfy an increased measure of safety. Formulated conversely, the measuring devices must offer a certain measure of reliability. This is described via the safety requirement stage or the safety integrity level (SIL). The risk of a malfunction must be further minimized with a higher stage.
In order to recognize malfunctions, measuring devices are provided with a functionality which makes it possible for them to monitor especially themselves or individual components.
If a fault is recognized, this is signaled, on the one hand, especially via a fault signal, and on the other, the measuring devices, especially for critical faults, change into a protected state which is designed to prevent an accident from occurring by the measuring device in conjunction with the fault state.
For the further examination, measuring devices or in general measuring arrangements are divided into at least one sensor device and one transmitter device. The sensor device based on a measurement generates a measurement signal which is processed by the transmitter device. The processing can mean, for example, that in the transmitter device calibration data are stored which make it possible to deduce from the measurement signal the measured quantity of actual interest. If for example, the measurement signal is the transit time of a microwave signal of a liquid level measuring device according to the radar principle, the level of a medium which reflects the microwave signal can be deduced via reference data. In addition or alternatively, the measurement signals are converted in the transmitter device into generally standardized data which can be communicated via field busses. The generated output signal is, for example, a 4 . . . 20 mA signal.
A measuring arrangement quite generally is to be understood both as a measuring device—an alternative designation would be: field device—and also an interconnection of separate components for a measurement.
In addition to detecting a fault, it is also important for the measuring arrangement to pass reliably into a safe state for such a fault case, i.e., especially change into a rest state or an off state.